It is well known in the art of railway rolling stock that the phenomenon known as "train action" occurs more or less continuously throughout the length of a moving train of railway cars. Train action results when relative acceleration and deceleration between adjacent cars in the train subjects the draft gear and coupler assemblies at the ends of each car to short longitudinal motion cycles between fully extended and fully compressed limits. Typical friction draft gear assemblies generally will accommodate 2-3/4 inches to b 4-1/2 inches of longitudinal travel from no load to the extreme buff or compression condition or from no load to the extreme draft or tension condition for a total coupler travel of 5-1/2 inches to 9 inches. This relative motion occurs continuously between adjacent railway cars as the respective couplers and draft gears respond to the relative accelerations and decelerations of the cars.
In general, such relative accelerations and decelerations between adjacent cars can occur at any point in a moving train; however, perhaps the most common and conspicuous example of train action is the transmission of a starting or a braking impulse from the locomotive in rapid sequence through the length of the train. Train action thus occurs when a train is accelerating from a dead stop in either the forward or reverse direction, decending a downgrade or passing from a downgrade to an upgrade, for example.
When the longitudinal coupler forces between two railway cars approach zero, the draft gears in each of the respective couplers extend to their fully extended limit under the bias of the longitudinal spring elements incorporated therein. The subsequent application of longitudinal force to the car couplers, either in tension (draft) or compression (buff), tends to overcome the extension bias of the draft gear spring elements. With application of buff or draft loads of sufficient magnitude, the draft gears will be compressed to the limits of longitudinal motion imposed by the draft gear structure.
When moving from a buff or compression condition to a draft or tension condition each affected draft gear experiences a complete motion cycle, moving from a state of compression to extension, to compression again. Similarly, for every complete force cycle between adjacent cars from buff loading to draft loading to buff loading again, both of the affected draft gears undergo two complete motion cycles. The compression-to-extension-to-compression cycle is completed once as the cars move from a buff to a draft condition, and a second such cycle is completed as the cars are returned from draft condition to the buff condition.
As can be seen, the relative acceleration and deceleration between the cars of a train results in considerable relative longitudinal movement of the draft gear and coupler components, all of which are subject to wear as a result of such relative movement. In particular, a draft gear assembly commonly includes a coupler follower block located intermediate the coupler and the draft gear. The coupler follower block transmits buff loads from the coupler directly to the draft gear while coupler tension forces are transmitted in the well known manner from the coupler to the yoke to the draft gear and finally via the coupler follower block to the draft sill stops. Thus, the coupler follower block is substantially continuously lodged against the draft sill stops when the coupled cars are in draft, or when the longitudinal force between them is nil. However, when the coupler forces between adjacent cars change from nil to a draft condition and back to nil, the draft gear yoke moves longitudinally with respect to the stationary follower block as the draft gear first compresses and then re-extends.
The follower block also experiences a complete longitudinal motion cycle whenever the coupler forces go from nil to buff. In response to buff loading, the follower block moves with respect to the center sill throughout a range of motion dictated by the range of draft gear compression from its extended condition. For coupler forces approaching an extreme buff condition, the follower block moves longitudinally with respect to the center sill to its motion limit which is dictated by the solid compressed condition of the draft gear. During the range of coupler forces from extreme buff to nil, the draft gear re-extends to its free state thus moving the follower block longitudinally with respect to the center sill to the limit defined by engagement thereof with the draft sill stops.
Of course, as the coupling assemblies for railway cars are not produced to close tolerances, there is always the potential for misalignment of coupler and draft gear assembly elements. Furthermore, there is always the potential for misalignment between adjacent cars or the couplings thereof in a train. For example, when traversing a lateral track curve the adjacent cars of a train are necessarily misaligned and the draft or buff forces between the cars therefore also are out of alignment with the center sill and coupling system center line. Thus, there is considerable free play in known coupling components, especially transverse free play, and this is aggravated over time by progressive wear of the coupling components. As a result, misalignment of draft and coupling components may arise quite readily even when a train is travelling on tangent track rather than negotiating a curve. Since the draft and buff forces which are commonly imposed on the draft gear and coupling components of a railway car may exceed one million pounds, even very small magnitudes of longitudinal misalignment may result in transverse force components of considerable magnitude on draft and coupler elements.
As will be appreciated from the above, during changes in magnitude and/or direction of car coupling forces, there is continuing relative motion between the coupler follower block, center sill, and yoke surfaces in a draft gear assembly. Due to the inevitable misalignment as above described, the draft and buff coupling forces often may be misaligned with the longitudinal center line of the car couplings. Hence, transverse force components may be imposed upon the relatively longitudinally movable elements, most notably the follower block, and this can lead to undue wear resulting from considerable frictional rubbing between the relatively longitudinally moving members under the applied tranverse load components.
There have been prior attempts to alleviate this wear condition, including attempts to line the inner sidewall surfaces of the center sill with a liner material. This approach has thus far proven to be unsatisfactory as the longitudinal scraping of the follower block along the center sill sidewalls under significant transverse loads destroys the liner very quickly in service.